The present invention relates to a control system for an automatic clutch such as an electromagnetic clutch for a transmission of a motor vehicle, and more particularly to a system for controlling the clutch when starting the motor vehicle.
Recently, an automatic clutch has been employed in an motor vehicle for smoothly transmitting power of an engine to the transmission. The automatic clutch of an automatic transmission is controlled by a control system to provide various operational modes for clutch torque. One of the modes is selected in accordance with a position of a selector lever and detected driving conditions. The clutch torque is increased with engine speed and time. At fast-idle of the engine during cold engine operation or during operation of an air conditioner, the clutch torque is corrected to slowly engage the clutch.
There is an automatic transmission having a racing start mode which is automatically selected in accordance with the selection of a drive range from a neutral range at high engine speed. Therefore, when the vehicle starts under the aforementioned condition, the racing start mode prevents a shock. More particularly, in the start mode, the engine races, and then the clutch is engaged to rapidly start the vehicle. If the clutch is slowly engaged in the same manner as an ordinary starting mode, the clutch torque at an initial stage of the starting is too small to start the vehicle, so that the engine speed is excessively increased, causing damage to the engine. Furthermore, the shock caused by inertia force at the high engine speed is exerted on the transmission, so that the power transmission system is subjected to an overload. In a continuously variable belt-drive transmission, the belt may slip with the overload. Therefore, it is necessary to properly control the clutch torque at the racing start.
Japanese Patent Application Laid-open 61-220942 discloses a clutch control system. The system comprises a first function system for determining clutch pressure of a clutch in accordance with functions of engine speed and accelerator pedal depressing degree, a second function system for determining the clutch pressure in accordance with functions of the accelerator pedal depressing degree and time, and a control unit for controlling the clutch pressure in accordance with the determined clutch pressure. In the racing start mode, the second function is selected based on high engine speed, thereby slowly increasing the clutch pressure in accordance with a function of time, thereby reducing the shock at the starting of the vehicle.
In the conventional system, however, since the increasing rate of the clutch pressure is regulated for reducing the shock at the engagement of the clutch, the engine speed is unnecessarily increased and hence the starting performance is low. In particular, in the case of the electromagnetic clutch, the response time of the clutch current to the clutch current control signal is comparatively long so that the increase of the clutch torque with an increase of the engine speed is delayed. Therefore, the engine speed is quickly increased and the clutch is engaged at a high engine speed. Thus, a large shock occurs with a large engine torque and a large inertia mass.